Whispering-Gallery Mode (WGM) optical microcavities are rich experimental platforms for quantum optics, photonics, and sensing. Ultrahigh-Quality factor (Q) toroidal optical microcavities are particularly suited for nonlinear optics and single particle detection, due to their combination of extremely narrow linewidths and small mode volumes. Sensing applications typically entail detection of analyte binding via small shifts in the resonance wavelength, resonator Q-factor, or mode splitting. One advantage of the toroidal geometry is the simplified mode structure relative to other microresonator geometries, which leads to widely separated resonances. However, the high finesse (106) presents a significant experimental difficulty. The absolute resonant wavelengths are determined by minor variations in the fabrication conditions. In particular, laser-induced reflow, a crucial fabrication step, relies upon a runaway thermal process, although larger diameter toroids have been produced without this step. Typically, tunable external-cavity diode lasers are used to couple light into the resonator, as they possess narrow linewidth and wavelength tunability greater than the resonator's free spectral range (FSR). However, the need for expensive tunable light sources impedes applications that require low-cost fabrication or integration into massively parallel device architecture.
Progress has been made in controlling absolute resonance position in WGM microcavities. These efforts include chemical etching and UV irradiation, though these processes are irreversible, and have not been demonstrated with microtoroids. The entire silicon chip can be heated, although this approach cannot achieve independent control of multiple toroids and is limited by slow switching speeds. A photochromic thin film can be optically pumped to photothermally heat the resonator, but is slowed by the recovery time of the molecule's ground state (˜11 s). (See J. Topolancik and F. Vollmer, Appl. Phys. Lett. 89, 184103 (2006).) Similarly, a highly absorbing polymer film can be applied to a microsphere and optically pumped, but with limited tuning range (25 pm) and speed (165 ms). (See H. C. Tapalian, J. P. Laine, and P. A. Lane, IEEE Phot. Tech. Lett. 14, 1118 (2002).) Toroids can be individually functionalized with a heating element, but this approach increases fabrication complexity. (See D. Armani, B. Min, A. Martin, and K. J. Vahala, Appl. Phys. Lett. 85, 5439 (2004).) A second fiber-coupled light source can be used to photothermally tune the resonance, but requires high powers due to the transparency of fused silica at the pump wavelength and is consequently limited to a narrow tuning range. The need for two tunable narrow-linewidth sources also adds cost and complexity. (See J. G. Zhu, S. K. Ozdemir, L. N. He, and L. Yang, Appl. Phys. Lett. 99, 171101 (2011).)